This filter’s unusually low DC offset is due to the use of an OPA380 at U1. While this amplifier is usually employed as a transimpedance amplifier, it can also be used as an inverting op amp. In this configuration it provides wide bandwidth with extremely low input offset voltage and drift.
The OPA380 is a monolithic 85MHz GBW CMOS op amp with an internal auto- zeroed integrator. This integrator forces the high-speed op amp input offset and drift to virtually zero.
This 100kHz “Low-Pass Filter With Very Low DC Offset” circuit is a two- pole multiple- feedback filter with a Butterworth response. This filter has a DC gain of 1V/V or 0dB. Above the -3dB corner frequency its response is close to theoretical up to 10MHz; above this frequency the finite GBW of U1 prevents much additional filter rolloff. An OPA380 is not suitable for a Sallen- Key active filter; that topology requires an op amp to be used as a non-inverting amplifier. The OPA380’s non-inverting input is a very low bandwidth integrator input. (Circuit is created by Neil P. Albaugh, TI – Tucson )
“Low-Pass Filter With Very Low DC Offset” Circuit:
Online Simulation of the “Low- Pass Filter With Very Low DC Offset” Circuit
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